A wide range of medical treatments exist that utilize “endoluminal prostheses.” As used herein, endoluminal prostheses is intended to cover medical devices that are adapted for temporary or permanent implantation within a body lumen, including both naturally occurring and artificially made lumens, such as without limitation: arteries, whether located within the coronary, mesentery, peripheral, or cerebral vasculature; veins; gastrointestinal tract; biliary tract; urethra; trachea; hepatic shunts; and fallopian tubes.
Accordingly, a wide assortment of endoluminal prostheses have been developed, each providing a uniquely beneficial structure to modify the mechanics of the targeted lumen wall. For example, stent prostheses are known for implantation within body lumens to provide artificial radial support to the wall tissue, which forms the various lumens within the body, and often more specifically, for implantation within the blood vessels of the body.
Essentially, stents that are presently utilized are made to be permanently or temporarily implanted. A permanent stent is designed to be maintained in a body lumen for an indeterminate amount of time and is typically designed to provide long term support for damaged or traumatized wall tissues of the lumen. There are numerous conventional applications for permanent stents including cardiovascular, urological, gastrointestinal, and gynecological applications. A temporary stent is designed to be maintained in a body lumen for a limited period of time in order to maintain the patency of the body lumen, for example, after trauma to a lumen caused by a surgical procedure or an injury.
Permanent stents, over time, may become encapsulated and covered with endothelium tissues, for example, in cardiovascular applications, causing irritation to the surrounding tissue. Further, if an additional interventional procedure is ever warranted, a previously permanently implanted stent may make it more difficult to perform the subsequent procedure.
Temporary stents, on the other hand, preferably do not become incorporated into the walls of the lumen by tissue ingrowth or encapsulation. Temporary stents may advantageously be eliminated from body lumens after an appropriate period of time, for example, after the traumatized tissues of the lumen have healed and a stent is no longer needed to maintain the patency of the lumen. As such, temporary stents may be removed surgically or be made bioabsorbable/biodegradable.
Temporary stents may be made from bioabsorbable and biodegradable materials that are selected to absorb or degrade in vivo over time. However, there are disadvantages and limitations associated with the use of bioabsorbable or biodegradable stents. Many bioabsorbable or biodegradable materials have insufficient mechanical strength or other properties, such as a lack of plastic deformability, to perform adequately under the loading conditions within a treatment location upon implantation. Other limitations arise in controlling the break down of the bioabsorbable materials from which such stents are made, as in, preventing the material from breaking down too quickly or too slowly. If the material is absorbed too quickly, the stent will not provide sufficient time for the vessel to heal, or if absorbed too slowly, the attendant disadvantages of permanently implanted stents may arise.
In order to obtain the ideal properties for a bioresorbable vascular implant, the implant must have sufficient mechanical properties to withstand the loading conditions within the treatment location upon implantation. This requires a high degree of mechanical strength initially upon acute implantation to resist forces exerted on the implant by the surrounding tissue. However, due to the dynamic healing response of the body these forces decrease over time as the surrounding tissue remodels. This remodeling process begins immediately after implantation and may continue for several months, even years. From a stent design viewpoint, there is a point in time at which the remodeling process has achieve sufficient mechanical scaffolding to prevent undesirable clinical recoil or collapse without the need of a metallic implant. It is therefore desirable to have a vascular implant with a high degree of mechanical strength initially, but diminishing strength as time proceeds. Accordingly, there is a need for a temporary stent that provides sufficient support in a body lumen for the duration of a therapeutically appropriate period of time, which then degrades to be eliminated from the patient's body without surgical intervention.